Method for coating sliding surface of high-temperature member, high-temperature member and electrode for electro-discharge surface treatment

ABSTRACT

A sliding surface of a high-temperature portion is subjected to an electro-discharge surface treatment with one or both of a high-temperature hard material ( 4 ) and a material having a lubricating property at a high temperature ( 6 ). The high-temperature hard material ( 4 ) is any or a mixture of cBN, TiC, TiN, TiAlN, TiB 2 , WC, Cr 3 C 2 , SiC, ZrC, VC, B 4 C, Si 3 N 4 , ZrO 2 , and Al 2 O 3 . The material having the lubricating property at the high temperature ( 6 ) contains chromium and/or chromium oxide (Cr 2 O 3 ) and/or hexaboron nitride (hBN). An electrode formed by compression molding of the high-temperature hard material, and the high-temperature lubricating material containing at least one of Cr and hBN and having the lubricating property at the high temperature is used as the electrode for the electro-discharge surface treatment.

This is a Continuation-in-Part Application in the United States of International Patent Application No. PCT/JP03/12088 filed Sep. 22, 2003, which claims priority on Japanese Patent Application No. 295963/2002, filed Oct. 9, 2002 and Japanese Patent Application No. 276860/2002, filed Sep. 24, 2002. The entire disclosures of the above patent applications are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Technical Field of the Invention

The present invention relates to a method for coating a sliding surface of a high-temperature member, the high-temperature member, and an electrode for an electro-discharge surface treatment.

2. Description of the Related Art

FIG. 1 is a schematic diagram of a shrouded turbine blade as an example of a high-temperature member seen from a shroud portion, and FIG. 2 is another perspective view of a shroud section.

The shrouded turbine blade is constituted of a blade portion 1, a dove tail portion 2, and a shroud portion 3. The blade portion 1 is a portion which has a blade-shaped section and in which a rotation power is generated by combustion gas. The dove tail portion 2 is positioned in a terminal end portion of the blade portion 1, and is fixed to a turbine disk (not shown) to transmit the rotation power to the turbine disk. The shroud portion 3 is attached to a blade tip end, and has a function of suppressing vibration or reducing a gas leakage in the tip end.

As shown in this figure, the shroud portions are formed to be integrated with one or a plurality of blade portions 1, and are assembled to closely contact with one another. The closely contact surface (abutment surface) is simply linear as seen from a radial direction in some case, and is stepped midway in the other case as in this example. The surface stepped midway will hereinafter be referred to as “Z-notched”.

For a Z-notched shroud portion, shroud portions disposed adjacent to each other as shown by a two-dot chain line in FIG. 1 abut on each other on side surfaces A, B of a Z notch 3 a to hold the entire position. Therefore, the Z-notched shroud portions have characteristics that the portions have a high position hold capability without being connected to each other.

However, the turbine blade rotate at a high speed during operation, and not only undergoes periodic deformation and vibration but also is exposed to the combustion gas at a high temperature passed through the turbine blade. Therefore, there is a problem that the side surfaces A, B of the shroud portion receive a high surface pressure at the high temperature while rubbing, and are severely abraded. It is to be noted that the sliding surfaces A, B are not fixed, and one surface is defined as A while the surface abutting on the one surface is defined as B.

To solve the problem, a hard heat-resistant/wear-resistant metal has heretofore been build-up welded or thermally sprayed to the surfaces disposed adjacent to the Z notch of the turbine blade and sliding against each other (side surfaces A, B, hereinafter referred to as “sliding surfaces A, B”) (e.g., [Reference 1]).

Reference 1

-   Japanese Laid-Open Patent Publication No. 2001-152803

However, for the build-up welding and thermally spraying, a layer forming rate is high, but there are problems that soundness/adhesion of layers, dimensional precision, and workability are bad, and automation is difficult. There are also problems that a pre-treatment/post-treatment are necessary and cost is high.

On the other hand, as shown in FIGS. 3 and 4, a large number of fitting portions 5 are used in a turbine high-temperature section. As shown in FIG. 5, a large number of sliding surfaces exist even in a stator blade segment of a compressor. A rear stage of the compressor has a high temperature during the operation.

The fitting portions of the turbine high-temperature section and the sliding surfaces of the compressor rub against each other during the operation and are therefore easily worn, and need to be coated with a high-temperature wear-resistant material. Therefore, a hard material has heretofore been attached to the surface by welding or thermal spraying. That is, a surface layer of a component is activated with grinding or blast before the welding or the thermal spraying. Subsequently, a Stellite-based alloy is built up by the welding or the thermal spraying, and grinding processing is carried out to remove an excess metal and to secure dimensions after the welding or the thermal spraying. However, there are problems that the fitting portions are narrow trenches, the welding or the thermal spraying is not easily carried out, and the portions of conventional materials are easily worn.

On the other hand, a technique for coating the surface of the metal material by in-liquid electro-discharge to impart corrosion and wear resistances has already been applied for a patent and well known. Main points of this technique are as follows. First, powders of tungsten carbide (WC) and Co are mixed, compressed, and molded into an electrode, the in-liquid electro-discharge is carried out with the electrode, and an electrode material is deposited on a work. Thereafter, re-melting electro-discharge processing is carried out on the electrode material deposited on the work by another electrode (e.g., a copper electrode, graphite electrode, and the like), and a higher hardness and adhesion are obtained in this method. This related art will hereinafter be described.

First, a second related art will be described (e.g., refer to Reference 2). A mixed green compact electrode of WC-Co is used to perform the electro-discharge processing on the work (mother material S50C) in the liquid and to deposit WC-Co (primary processing). Subsequently, re-melting processing (secondary processing) is carried out by an electrode which is not worn out very much such as a copper electrode. Only with the deposition of the primary processing, a coat layer has a hardness (Vickers hardness) of about Hv=1410 and includes a structure in which there are many hollows. However, by the secondary processing which is the re-melting processing, the hollows are eliminated, and a coat layer having an improved structure indicating a hardness of Hv=1750 is obtained. In accordance with the method of the second related art, a hard coat layer having good adhesion is obtained with respect to a steel material. However, it is difficult to form the coat layer having a strong adhesion on the surface of a sintered material such as a sintered hard alloy.

Next, in accordance with a third related art by the present applicant, it has been clarified that the material forming hard carbide such as Ti is used as the electrode to generate the electro-discharge between the electrode and a metal material constituting the work, and then a strong hard coating layer can be formed on the surface of the metal as the work without any re-melting process (e.g., see Reference 3). This is because the electrode material worn by the electro-discharge reacts with carbon C that is a component in dielectric liquid to generate titanium carbide (TiC). Furthermore, it has been found that when the electro-discharge is generated between the metal material of the work and the green compact electrode of hydride of the metal such as titanium hydride (TiH₂), the hard coating layer having good adhesion can be formed more quickly as compared with the use of the material such as Ti. It has further been found that when the electro-discharge is generated between the metal material of the work and the green compact electrode obtained by mixing hydride such as TiH₂ with another metal or ceramic, a hard coating layer having various properties such as hardness and wear resistance can quickly be formed.

Moreover, in accordance with a fourth related art, it has been found that a surface treatment electrode having a high strength can be prepared by preliminary sintering (e.g., see Reference 4). As one example of the fourth related art, the preparation of the electrode for the electro-discharge surface treatment, constituted of a mixed powder of WC and Co powders will be described. For the green compact obtained by mixing, compressing, and molding the WC and Co powders, the WC and Co powders may simply be mixed, compressed, and molded. However, when the powders are compressed/molded after mixing wax, moldability of the green compact is more preferably improved. However, the wax is an insulating material. Therefore, when a large amount of wax is left in the electrode, electric resistance of the electrode increases, and therefore electro-discharge properties are degraded. Therefore, it is necessary to remove the wax. When the green compact electrode is charged and heated in a vacuum furnace, the wax can be removed. At this time, when the heating temperature is excessively low, the wax cannot be removed. When the temperature is excessively high, the wax turns to soot, and purity of the electrode is degraded. Therefore, the heating temperature needs to be kept at a temperature which is not less than a temperature to melt the wax and which is not more than a temperature to decompose the wax and to turn to soot. Next, the green compact in the vacuum furnace is heated by a high-frequency coil and calcined to such an extent that a strength can bear machining, such that the compact is not excessively hardened, and exhibits a hardness like that of chalk. However, a contact portion between carbides is calcined at a comparatively low sintering temperature such that coupling is advanced but weak coupling is achieved before real sintering. It is known that when the electro-discharge surface treatment is carried out with this electrode, a dense and homogeneous coat can be formed.

Next, in accordance with a fifth related art, it has partially been seen that various functions can be imparted to the coating layer by adjustment of materials to be blended as the electrode materials (e.g., see Reference 5). In this fifth related art, it is disclosed that lubricating properties can be imparted to the coating layer by mixture of a material indicating the lubricating properties with the electrode.

Moreover, in a method disclosed in a sixth related art, the work is used as a cathode, any of W or WC, Stellite-based alloy, TiB₂ (titanium boride), and Cr₃C₂ (chromium carbide) is formed in a rod-shaped electrode to carry out the electro-discharge processing, and fixing layers such as W or WC, Stellite-based alloy, TiB₂, and Cr₃C₂ are formed on the surface of the work. Thereafter, W or WC, Cr₃C₂, Co, Cr, Al, Y, and the like are thermally sprayed to the surface of the fixing layer, and thereafter the surface is further subjected to plasma processing to obtain the wear resistance (e.g., see Reference 6).

Next, in a seventh related art, an electrode for the electro-discharge processing, formed of Cr₃C₂ and the like to inhibit the wear on the electrode is disclosed (e.g., see Reference 7). In a method disclosed in an eighth related art, the materials such as WC, TaC, TiC, cBN (cubic boron nitride), and diamond are used as the electrode material to attach the electrode material melted by the electro-discharge in the atmosphere to the work, and the coating layer is formed (e.g., see Reference 8). Furthermore, a ninth related art relates to an electro-discharge surface treatment technique, and lubricating materials such as BN are added to the electrode for the electro-discharge surface treatment to impart a lubricating function to the coating layer (e.g., see Reference 9).

Reference 2

-   Japanese Laid-Open Patent Publication No. 5-148615 (pages 3 to 5)

Reference 3

-   Japanese Laid-Open Patent Publication No. 9-192937 (page 9)

Reference 4

-   WO99/58744 (pages 18 to 20)

Reference 5

-   WO00/29157 (pages 6 to 7)

Reference 6

-   Japanese Laid-Open Patent Publication No. 8-81756 (pages 2 to 3)

Reference 7

-   Japanese Laid-Open Patent Publication No. 3-66520 (page 2)

Reference 8

-   Japanese Laid-Open Patent Publication No. 8-53777 (page 3)

Reference 9

-   Japanese Laid-Open Patent Publication No. 2001-279465 (pages 4 to 5)

However, in some of the electro-discharge surface treatments disclosed in the above-described second to ninth related arts, the lubricating materials are added to form a functional coating layer, but in most of the arts, the wear resistance at room temperature is focused on, and therefore the coating layer of the hard material such as TiC is formed on the surface of a material to be processed.

On the other hand, in recent years, there has been a strong demand for a coating layer which has wear resisting properties under a high-temperature environment or lubricating properties. FIG. 6 shows a schematic diagram of a turbine blade of a gas turbine engine for an airplane. In this gas turbine engine for the airplane, as shown in the figure, a plurality of turbine blades 201 contact one another and are fixed, and are constituted to rotate around an axis. When the turbine blades 201 rotate during the operation of the gas turbine engine for the airplane, a contact portion A among the shown turbine blades 201 is extremely rubbed and hit under a high-temperature environment. There has been a problem that the wear-resistant coating layer used in the above-described related arts is degraded in hardness or oxidized under the high-temperature environment (700° C. or more) under which the turbine blade is used, and therefore there is hardly wear-resistant effect. Moreover, the coating layer to which the lubricating properties are imparted by the fourth and eighth related arts is based on an assumption of the use at room temperature. The lubricating properties at room temperature is far different in phenomenon and mechanism from those under the high-temperature environment exceeding 700° C. for use in the gas turbine engine for the airplane. These related arts have a problem that the lubricating properties in the high-temperature environment are not considered.

SUMMARY OF THE INVENTION

The present invention has been developed to solve the above-described problems. That is, a first object of the present invention is to provide a method for coating a sliding surface of a high-temperature member and the high-temperature member which is little worn even by sliding movement under a high surface pressure at a high temperature, in which adhesion, dimensional precision, and workability of layers are improved and automation is possible.

Moreover, a second object of the present invention is to provide an electrode for an electro-discharge surface treatment by which a coating layer having wear resistance and lubricating property even under a high-temperature environment can be formed on a workpiece by an electro-discharge surface treatment method.

To achieve the first object, according to the present invention, there is provided a method for coating a sliding surface of a high-temperature member, comprising the steps of: using an electrode for an electro-discharge treatment of a green compact of a high-temperature hard material (4), a material (6) having a lubricating property at a high temperature, or a mixture of the high-temperature hard material (4) and the material (6) having the lubricating property at the high temperature on a sliding surface which contacts another member under a high-temperature environment to generate a pulsed electro-discharge between the electrode and the sliding surface; and forming a coating layer of a constituting material of the electrode for the electro-discharge surface treatment or a reactant of the constituting material on the sliding surface by electro-discharge energy.

Further, according to the present invention, there is provided a method for coating a sliding portion, comprising the steps of: subjecting a sliding surface A which contacts another member under a high-temperature environment to an electro-discharge surface treatment with a high-temperature hard material (4), a material (6) having a lubricating property at a high temperature, or a mixture of the high-temperature hard material (4) and the material (6) having the lubricating property at the high temperature, the electro-discharge surface treatment being a treatment of generating a pulsed electro-discharge between an electrode for the electro-discharge surface treatment and a workpiece in dielectric liquid or gas, so that a constituting material of the electrode for the electro-discharge surface treatment or a reactant of the constituting material forms into a coating layer on the surface of the workpiece by electro-discharge energy; and subjecting a sliding surface B that will abut on the sliding surface A to a welding, thermal spraying, or electro-discharge surface treatment with the material (6) having the lubricating property at the high temperature.

According to the present invention, there is provided a method for coating a sliding portion, comprising the steps of: subjecting a sliding surface A which contacts another member under a high-temperature environment to an electro-discharge surface treatment with a high-temperature hard material (4), a material (6) having a lubricating property at a high temperature, or a mixture of the high-temperature hard material (4) and the material (6) having the lubricating property at the high temperature; and subjecting the sliding surface B that will abut on the sliding surface A to the electro-discharge surface treatment with the high-temperature hard material (4), the material (6) having the lubricating property at the high temperature, or the mixture of the high-temperature hard material (4) and the material (6) having the lubricating property at the high temperature.

Furthermore, according to the present invention, there is provided a method for coating a sliding surface, comprising the steps of: subjecting a sliding surface which contacts another member under a high-temperature environment to an electro-discharge surface treatment with a material (6) having a lubricating property at a high temperature in a porous manner (i.e., in a manner forming a porous coating layer); and filling pores with a solid lubricating material (7).

Preferably, the method further comprises the step of: to fill the pores with solid lubricating material, mixing the solid lubricating material in volatile solvent to brush-coat the material; or mixing the solid lubricating material in a soft rubber to be filled in the pores by rubbing.

Preferably, the solid lubricating material (7) is chromium oxide (Cr₂O₃), or a mixture of chromium oxide (Cr₂O₃) and silicon.

Preferably, the method further comprises the step of: performing a heating treatment after performing the electro-discharge surface treatment with the material having the lubricating property at the high temperature (6).

Preferably, the high-temperature hard material (4) is any one or a mixture of cBN, TiC, TiN, TiAlN, TiB₂, WC, Cr₃C₂, SiC, ZrC, VC, B₄C, Si₃N₄, ZrO₂, and Al₂O₃.

Preferably, the material (6) having the lubricating property at the high temperature contains chromium and/or chromium oxide (Cr₂O₃) and/or hBN.

Preferably, the material (6) having the lubricating property at the high temperature contains 10% or more of chromium, and does not contain 20% or more of nickel.

Furthermore, according to the present invention, there is provided a high-temperature member having a sliding surface which will contact another member under a high-temperature environment and on which a coating layer of a constituting material of an electrode for an electro-discharge surface treatment or a reactant of the constituting material is formed by the electro-discharge surface treatment in which the electrode of a green compact of a high-temperature hard material (4), a material (6) having a lubricating property at a high temperature, or a mixture of the high-temperature hard material (4) and the material (6) having the lubricating property at the high temperature is used to generate a pulsed electro-discharge between the electrode and the sliding surface, the sliding surface contacting another member under a high-temperature environment.

According to the present invention, there is provided a high-temperature member comprising: one sliding surface which will contact the other sliding surface on another member under a high-temperature environment and which is subjected to a electro-discharge surface treatment with a high-temperature hard material (4), a material (6) having a lubricating property at a high temperature, or a mixture of the high-temperature hard material (4) and the material (6) having the lubricating property at the high temperature, wherein the other sliding surface is welded, thermally sprayed, or subjected to the electro-discharge surface treatment with the material (6) having the lubricating property at the high temperature.

Furthermore, according to the present invention, there is provided a high-temperature member comprising: one sliding surface which will contact the other sliding surface on another member under a high-temperature environment and which is coated with a high-temperature hard material (4), a material (6) having a lubricating property at a high temperature, or a mixture of the high-temperature hard material (4) and the material (6) having the lubricating property at the high temperature, wherein the other sliding surface is subjected to an electro-discharge surface treatment with the high-temperature hard material (4), the material (6) having the lubricating property at the high temperature, or the mixture of the high-temperature hard material (4) and the material (6) having the lubricating property at the high temperature.

According to the present invention, there is provided a high-temperature member comprising: a sliding surface which will contact another member under a high-temperature environment and which is subjected to an electro-discharge surface treatment with a material (6) having a lubricating property at a high temperature in a porous manner (i.e., in a manner forming a porous coating layer), wherein pores are filled with a solid lubricating material (7).

Preferably, the solid lubricating material (7) is chromium oxide (Cr₂O₃), or a mixture of chromium oxide (Cr₂O₃) and silicon (Si).

Preferably, after performing the electro-discharge surface treatment is performed with the material (6) having the lubricating property at the high temperature, a heating treatment is performed such that the sliding surface has a tensile strength of 40 MPa or more.

Preferably, the material (6) having the lubricating property at the high temperature contains chromium and/or chromium oxide (Cr₂O₃) and/or hBN.

Furthermore, according to the present invention, there is provided a gas turbine component or an axial flow compressor component comprising the high-temperature member which is a turbine rotor blade, a stator blade segment, a compressor stator blade segment, a combustor of a gas turbine, or a stationary component of the turbine or an afterburner.

According to the method and high-temperature member of the present invention, when the sliding surfaces A, B are subjected to the electro-discharge surface treatment with the high-temperature hard material (4), high-temperature hardness of the sliding surfaces A, B can be enhanced. When the sliding surfaces A, B is subjected to the electro-discharge surface treatment with the material (6) having the lubricating property at the high temperature, the lubricating property of the sliding surfaces A, B at the high temperature can be enhanced. Furthermore, when both the high-temperature hard material (4) and the material (6) having the lubricating property at the high temperature are used in the electro-discharge surface treatment, high-temperature hardness and high-temperature lubricating property can simultaneously be enhanced.

Moreover, when the material (6) having the lubricating property at the high temperature is used in the electro-discharge surface treatment in the porous manner, and the pores are filled with the solid lubricating material (7), a stable lubricating property can be obtained from a low temperature of 700° C. or less to a high temperature of 700° C. or more. The material does not come to exhibit the lubricating property by oxidation at the high temperature, but the contained material itself has the lubricating property. Therefore, the material has the lubricating property even at 700° C. or less, and is not easily worn.

Furthermore, after the material (6) having the lubricating property at the high temperature is used in the electro-discharge surface treatment to form the coating layer (100) on the sliding surface so as to include pores (120), the material is subjected to the heating treatment, the porous structure of the coating layer can be densified and hardened, and a coating inner strength can be enhanced in a thick coating. Moreover, in the electro-discharge surface treatment, a tendency to an amorphous metal structure is seen, but the structure is crystallized and stabilized by the heating treatment, and a change of a dimension at the time of operation can be prevented.

Additionally, in the electro-discharge surface treatment, the coating layer has an adhesion equal to that of the welding, dimensional precision and quality are stable, workability is high, and cost is remarkably low as compared with the welding or thermal spraying. Therefore, a manufacturing cost can largely be reduced.

Moreover, the electro-discharge surface treatment can easily be applied to narrow trench-lie portions such as a fitting portion only by the change of an electrode shape, and a solid heat-resistant/wear-resistant coating layer can easily be formed.

According to a preferable aspect of the present invention, the high-temperature hard material (4) is any one or a mixture of cBN, TiC, TiN, TiAlN, TiB₂, WC, Cr₃C₂, SiC, ZrC, VC, B₄C, Si₃N₄, ZrO₂, and Al₂O₃.

Since these high-temperature hard materials have a very high Vickers hardness at room temperature as described later, and hold a high hardness even at the high temperature, the high-temperature hardness of the sliding surfaces A, B of the high-temperature member can largely be enhanced.

The material (6) having the lubricating property at the high temperature contains chromium and/or chromium oxide (Cr₂O₃) and/or hBN. The material (6) having the lubricating property at the high temperature preferably contains 10% or more of chromium, and does not contain 20% or more of nickel.

Chromium is oxidized at the high temperature to produce oxide, and fulfills the lubricating property. Therefore, a chromium-containing coating layer is formed on the sliding surfaces A, B of the high-temperature member to enhance the lubricating property at the high temperature, and the wear resistance can be enhanced equally or more as compared with the related art. When much nickel is contained, oxidation of chromium at the high temperature is inhibited.

As shown in FIGS. 7, 8, when a content of chromium is 10% or less, there is little chromium oxide, and a coefficient of friction is large. When the content of nickel is 20% or more, chromium oxide is not easily formed, and the coefficient of friction is large. Since chromium oxide and hBN themselves have the lubricating property, the lubricating property is indicated even at a low temperature.

The solid lubricating material (7) with which the pores (120) of the porous coating layer (100) are filled, as shown in FIG. 16, is preferably a material of chromium oxide, or a mixture of powders of chromium oxide and silicon. Instead of being oxidized for the lubricating property at the high temperature, the contained material itself has the lubricating property. Therefore, the material has the lubricating property at 700° C. or less, and is not easily worn.

To densify and harden the porous structure by the heating treatment, the material may be heated at 1000° C. for ten minutes or more. Accordingly, a tensile strength of about 100 MPa can be obtained, and peel in a porous layer can be prevented.

Moreover, to achieve the second object, according to the present invention, there is provided an electrode for an electro-discharge surface treatment for use in an electro-discharge surface treatment method of generating a pulsed electro-discharge between the electrode for the electro-discharge surface treatment and a workpiece in dielectric liquid or gas, so that a constituting material of the electrode for the electro-discharge treatment or a reactant of the constituting material forms into a coating layer on the surface of the workpiece by electro-discharge energy, wherein the electrode is made from a green compact of a mixture of a fine powder of a high-temperature hard material having a high hardness at a high temperature, and a fine powder of a high-temperature lubricating material which is oxidized at the high temperature to obtain a lubricating property.

Accordingly, the electrode for the electro-discharge surface treatment made from a mixture of a high-temperature hard material having a high strength at the high temperature, and a high-temperature lubricating material having the lubricating property at the high temperature is used to form a coating layer which is not easily worn even under a high-temperature environment at 700° C. or more on the surface of the workpiece by the electro-discharge surface treatment method.

Further, according to the present invention, there is provided an electrode for an electro-discharge surface treatment for use in an electro-discharge surface treatment method of generating a pulsed electro-discharge between the electrode for the electro-discharge surface treatment and a workpiece in dielectric liquid or gas, so that a constituting material of the electrode for the electro-discharge surface treatment or a reactant of the constituting material forms into a coating layer on the surface of the workpiece by electro-discharge energy, wherein the electrode is made from a high-temperature lubricating material which is oxidized at a high temperature to obtain a lubricating property.

Accordingly, the coating layer which exhibits the lubricating property under a high-temperature environment at 700° C. or more and which is not easily worn is formed on the surface of the workpiece formed by the electro-discharge surface treatment method using the electrode for the electro-discharge surface treatment made from the high-temperature lubricating material having the lubricating material at the high temperature. Moreover, the coating layer which exerts the lubricating property even under a low-temperature environment at 700° C. or less and which is not easily worn is formed on the surface of the workpiece formed by the electro-discharge surface treatment method using the electrode for the electro-discharge surface treatment including the solid lubricating material itself.

Moreover, when a solid lubricating material is filled in the pores of a porous coating layer of the workpiece formed by the electro-discharge surface treatment method using the electrode for the electro-discharge surface treatment made from the high-temperature material having the lubricating property at the high temperature, the coating layer which fulfills the lubricating property and is not easily worn even under the low-temperature environment at 700° C. or less is formed.

Other objects and advantageous characteristics of the present invention would be apparent from the following description with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a shrouded turbine blade seen from a shroud side;

FIG. 2 is a perspective view of a shroud portion;

FIG. 3 is a schematic diagram of a fitting portion of a turbine high-temperature section;

FIG. 4 is a schematic diagram of the fitting portion of a turbine nozzle;

FIG. 5 is a schematic diagram of the fitting portion of a compressor stator blade;

FIG. 6 is diagram showing a part of a gas turbine engine of an airplane;

FIG. 7 is a diagram showing a change of a coefficient of friction at a high temperature by a content of Cr;

FIG. 8 is a diagram showing an increase of friction at the high temperature by the content of Ni;

FIG. 9 is a schematic diagram of an electro-discharge surface treatment according to the present invention;

FIG. 10 is a diagram showing comparison of a wear amount of an Ni base with that of a Co base;

FIG. 11 shows comparison of Vickers hardness at the high temperature;

FIG. 12 is a comparative diagram of a high-temperature oxidation amount;

FIG. 13 is a comparative diagram of an X-ray intensity of chromium oxide at the high temperature;

FIGS. 14A and 14B show X-ray analysis results;

FIG. 15 is a diagram showing a low coefficient of friction of Cr₂O₃ at the high temperature;

FIG. 16 is a schematic diagram showing that a solid lubricating material is pores of a porous structure;

FIG. 17 is a flowchart showing a preparation procedure of an electrode for the electro-discharge surface treatment according to the present invention;

FIG. 18 is a diagram showing a section of a molding unit for preparing the electrode for the electro-discharge surface treatment; and

FIG. 19 is a schematic diagram showing a constitution of an apparatus for performing the electro-discharge surface treatment.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Preferable embodiments of the present invention will hereinafter be described with reference to the drawings. It is to be noted that components common with the respective drawings are denoted with the same reference numerals, and duplicate description is omitted.

In a coating method of sliding surfaces of a high-temperature member of the present invention, one or both of sliding surfaces A, B of the high-temperature member is subjected to an electro-discharge surface treatment with one or both of a high-temperature hard material (4) and a material (6) having a lubricating property at a high temperature, but it is most preferable to subject both the sliding surfaces A, B of the high-temperature member to the electro-discharge surface treatment with both the high-temperature hard material (4) and the material (6) having the lubricating property at the high temperature.

FIG. 9 is a schematic diagram of the electro-discharge surface treatment according to the present invention. An electro-discharge surface treatment technique is disclosed, for example, in Japanese Laid-Open Patent Publication No. 7-197275 “Surface Treatment Method of Metal Material by In-Liquid Electro-discharge”. When an oxidation preventive measure and sensitivity of an electrically conductive/insulating servo are improved, the method may also be carried out even in gas.

In FIG. 9, reference numeral 8 denotes an electro-discharge electrode, 9 denotes dielectric liquid, 10 denotes a processing tank, 11 denotes an attachment member, and 12 denotes a power supply. In FIG. 9, a high-temperature member is fixed to the attachment member 11, and the electro-discharge electrode 8 is disposed opposite to the member in the vicinity of the sliding surface A (or B) of the high-temperature member. The dielectric liquid 9 is filled to such a height that at least the sliding surface A (or B) and a part of the electro-discharge electrode 8 are submerged. In this state, electro-discharge is generated between the electro-discharge electrode 8 and the sliding surface A (or B) by the power supply 12, and only the sliding surface A (or B) of the high-temperature member is coated.

It is to be noted that even for a narrow trench-shaped portion like a fitting portion of FIG. 3, only the sliding surface A (or B) can similarly be coated only by setting the shape of the electro-discharge electrode 8 so as to be fit for this trench.

In the electro-discharge surface treatment, when a voltage is applied to the high-temperature member and the electro-discharge electrode 8 immersed in the dielectric liquid or gas such as an argon gas and nitrogen, pulsed electro-discharge is generated between the surfaces disposed opposite to each other. The surface of the electro-discharge electrode 8 is peeled and melted by the electro-discharge, the melted material is attached to the sliding surface A (or B) of the shroud portion 3, but the treatment is most preferably carried out in oil.

Since a coating layer thickness is determined in proportion to the electro-discharge pulse number or a electro-discharge treatment time, and the conditions can be changed to control the thickness of the coating in several micrometers, the electro-discharge surface treatment is a coating method optimum for precision components such as a turbine motor blade. Moreover, the coating layer is formed only in a portion where the electro-discharge occurs, and therefore the coating layer can locally be formed on the portion to be coated, and pre-processing such as masking is unnecessary. Moreover, post treatments such as excessive thickness removal and dimension securing are also unnecessary. Heat input per pulse of the electro-discharge is remarkably small, and a turbine motor blade is hardly thermally deformed.

In the method of the present invention, as the electro-discharge electrode 8, powders of the high-temperature hard material 4 and/or the material having the lubricating property at the high temperature 6 are mixed, compressed, and molded for use. The high-temperature hard material 4 may also be any or a mixture of cBN, TiC, TiN, TiAlN, TiB₂, WC, Cr₃C₂, SiC, ZrC, VC, B₄C, Si₃N₄, ZrO₂, and Al₂O₃. TiC is preferable at a use temperature of the component of 900° C. or less, cBN is preferable at 900° C. or more and 1000° C. or less, and Al₂O₃, ZrO₂ are preferable at 1000° C. or more.

By the use of the high-temperature hard material (4) in the electrode for the electro-discharge surface treatment, the materials form a coating layer, and the sliding surface can be coated with a very hard fine ceramic (e.g., Vickers hardness is 4500 with cBN) at a low cost.

Moreover, the material (6) having the lubricating property at the high temperature indicates chromium, chromium oxide, hBN, or an alloy of cobalt containing these which is a major component. The chromium-containing material preferably contains 10% or more of chromium, and does not contain 20% or more of nickel.

When the material (6) having the lubricating property at the high temperature is used in the electrode for the electro-discharge surface treatment, the material forms the coating layer, and the lubricating property at the high temperature can be enhanced at the low cost.

Therefore, the processing cost largely decreases as compared with a conventional build-up welding or thermal spraying process.

When the chromium-containing material is used in the material (6) having the lubricating property at the high temperature, the lubricating property at the high temperature can be enhanced. This respect will be described in detail.

FIG. 10 is a diagram showing comparison of a wear amount of a chromium-containing Ni-based alloy with that of a Co-based alloy. From this figure, the wear amount drops for the cobalt-based alloy in a high-temperature range of 700 to 900° C. Additionally, it is seen that the wear amount rapidly increases in a high-temperature range for a nickel-based alloy.

FIG. 11 shows comparison of Vickers hardness at the high temperature. It is seen from this figure that Ni-based Inconel 713LC is relatively high in hardness even at the high temperature. However, although Ni-based Inconel 713LC is hard, the material is remarkably worn at the high temperature as described later.

FIG. 12 is a comparative diagram of a high-temperature oxidation amount. Although an Inconel material contains Cr, the material is not much oxidized at the high temperature, Cr₂O₃ is not formed, and low wear is not achieved. On the other hand, for the Co-based alloy, Cr is oxidized to form Cr₂O₃, and the low wear is supposedly achieved.

FIG. 13 is a comparative diagram of an X-ray intensity showing an amount of chromium oxide at the high temperature. In the chromium oxide amount produced on the surface of the metal, Co-based oxide rapidly increases at a temperature of 700° C. Cobalt does not inhibit the oxidation of chromium. On the other hand, even with a rise of temperature, the chromium oxide amount does not change and indicates a low value in the Ni-based alloy. From this, it is seen that a large difference is produced in the wear amount by the presence/absence of oxide as shown in FIG. 8.

FIGS. 14A and 14B show X-ray analysis results of the material after the heating at 900° C. In this figures, FIG. 14A shows Ni-based Inconel 713LC, and FIG. 14B shows the Co-based alloy (X-40). It is also seen from this comparison that Cr₂O₃ cannot apparently be formed in the Ni-based alloy.

Moreover, FIG. 15 is a diagram showing a coefficient of friction of Cr₂O₃ at the high temperature. It is seen from this figure that Cr₂O₃ indicates a low friction at the high temperature, and the coefficient of friction drops especially at around 300° C., and is remarkably small at around 700° C.

It is seen from the above-described characteristics of Cr₂O₃ that the lubricating property at the high temperature can be enhanced by the use of the chromium-containing material 6 having the lubricating property at the high temperature. Similarly, the lubricating property at the high temperature can also be enhanced by the use of the Cr₂O₃ or hBN-containing material 6 having the lubricating property at the high temperature.

The sliding surface A and/or B of the high-temperature member is subjected to the electro-discharge surface treatment with the material 6 having the lubricating property at the high temperature in the porous manner (i.e., in a manner forming a porous structure 100, namely, a coating layer having pores 120 as shown in FIG. 16), and the solid lubricating material 7 is mixed in volatile solvent, and the pores 120 are filled with the solid lubricating material 7 by brush coating, for example. Accordingly, the lubricating property can be enhanced even at 600° C. or less.

Moreover, diffusion coupling of the structure in the coating advances by the heating treatment, and densification and strengthening of the structure are realized.

The sliding surface A and/or B of the high-temperature member is subjected to the electro-discharge surface treatment with the material (6) having the lubricating property at the high temperature, the solid lubricating material (7) is mixed in the volatile solvent and filled in the pores (120) by the brush coating or mixed with a soft rubber and filled in the pores (120) by rubbing, and accordingly the lubricating property can be enhanced even at 700° C. or less. The porous structure (100) preferably has a porosity of 10% or more. As an example of the solid lubricating material (7), chromium oxide (Cr₂O₃) or a mixture of chromium oxide (Cr₂O₃) and silicon is used.

As one example, when the pores are filled with solid lubricating material Cr₂O₃, the lubricating property is imparted, and the material is not worn regardless of the temperature. When the solid lubricating material of Cr₂O₃ mixed with Si is filled in the pores, the wear is remarkably little at 480° C. Si is oxidized to form SiO₂, and liquefied glass and solid lubricating material Cr₂O₃ produce a synergistic effect.

Table 1 shows a test result indicating the effect of the solid lubricating material, and FIG. 16 is a schematic diagram showing that the solid lubricating material (7) is filled in the pores (120) of the porous structure (100).

TABLE 1 Example of test result of difference of wear at intermediate temperature Test conditions: 480° C. in the atmospheric air (after once heating at 760° C.) Surface pressure: 7 MPa Amplitude: 1.0 (±0.5) Cycle number: 10⁶ Wear amount (micron) Porous build-up of Stellite 31 190 Stellite 31 filled with Cr₂O₃ 50 Stellite 31 filled with Cr₂O₃ and Si 5

The porous structure has a small tensile strength and has a possibility of peel. To solve this, the structure is heated at the high temperature to promote diffusion among particles, and a coupling strength is increased.

Table 2 shows the test result of the tensile strength.

TABLE 2 Tensile strength MPa Electro-discharge coating as such 32 After high-temperature heat treatment 99

As one example, when the layer of the electro-discharge surface treatment is heated/treated at 1000° C. or more for ten minutes or more, the diffusion coupling of the structure in the coating advances, and the densification and strength enhancement of the structure are realized.

EXAMPLES

Examples of the present invention will hereinafter be described.

For the high-temperature material in the present invention, the turbine blade will be described in accordance with an example.

Table 3 is a major component table of a mother material (Inconel 713LC) of the turbine blade, a build-up material T onto the sliding surface by the welding, and a Stellite-based lubricating material which is a coating material onto the sliding surface by the electro-discharge surface treatment. The mother material has a small content of Cr and Co, but the material having a large content of these is used in the build-up material T and Stellite-based lubricating material. The build-up material T and Stellite-based lubricating material correspond to the material having the lubricating property at the high temperature 6.

TABLE 3 Material name Cr Co Mo Ni W Inconel 713LC 12 0 4.5 Balance — build-up material T 18 49 29 — — Stellite-based lubricating material 25 56 — 10 7

Table 4 shows the Vickers hardness of cBN and various types of carbides which are high-temperature hard materials at room temperature.

On the other hand, the mother material, build-up material T, and Stellite lubricating material of Table 3 have a low Vickers hardness of about 300 to 400. Additionally, it is known that the hardness rapidly drops at 600° C. or more, and at a high temperature of 700 to 800° C., the hardness drops to about ½ of that at room temperature.

TABLE 4 Vickers hardness (room temperature) cBN TiC SiC Cr₃C₂ WC ZrO₂ 4500 3200 2400 2280 2200 1300

Table 5 shows the test conditions for testing the wear on the sliding surface for confirming the effect of the present invention, and the conditions are set to those close to operation conditions of an actual jet engine.

TABLE 5 Surface Vibration Temperature pressure Cycle number number Amplitude 815° C. 7 MPa 1 × 10⁷ 95 Hz 0.35 mm

Table 6 shows comparison of the test results, and shows the test material and the degree of wear.

TABLE 6 No. Conditions Upper test piece Lower test piece 1   Mother materials Inconel 713LC Inconel 713LC 2   Lubricating build-up Build-up material T; Build-up material T; welds build-up welding build-up welding 3   Hard EDCs Cr₃C₂ + 5% Ni; EDC Cr₃C₂ + 5% Ni; (ceramic-based) EDC 4   Lubricating EDCs Stellite-based Stellite-based (Stellite-based) lubricating material; lubricating EDC material; EDC 5   Hard EDC and Cr₃C₂ + 5% Ni; EDC Build-up material T; lubricating build-up build-up welding welding 6-1 Hard EDCs (cBN) cBN (Ni plating cBN (Ni plating coat); EDC coat); EDC 6-2 Hard EDCs (TiC) TiC; EDC TiC; EDC 7   (Hard + lubricating) 50% Cr₃C₂ + 20% 50% Cr₃C₂ + 20% EDCs Cr + 30% Co; EDC Cr + 30% Co; EDC 8   (Hard + lubricating) 50% cBN + 20% 50% cBN + 20% EDCs Cr + 30% Co; EDC Cr + 30% Co; EDC Upper test piece Lower test piece No. wear amount wear amount Judgment 1 Large Large X 2 Excessively small Excessively small ⊚ but cost is high 3 Large Large X 4 Small Small ◯ cost is medium but thick build-up is possible and there is practical usability 5 Excessively small Excessively small ⊚ but cost is medium   6-1 Excessively small Excessively small ⊚ cost is low   6-2 Small Small ⊚ cost is excessively low 7 Medium Medium Δ 8 Excessively small Excessively small ⊚ cost is low

In this table, for treatment number 1, both materials are the mother materials, wear is excessively large, and this treatment cannot be applied. Treatment number 2 shows a conventional example (build-up welding), and the wear is little, but the cost is high. The treatment numbers 1 and 2 show known examples. However, the material is largely worn at 480° C. In Table 6, “conditions” indicates two frictional portions (surfaces) that contact each other to be worn in the experiment.

Treatment number 3 shows an example in which a hard ceramic-based material is used in the electrode, the electro-discharge surface treatment (described as electro-discharge coating (EDC) in Table 6) is applied, and hard chromium carbide (Cr₃C₂) having good oxidation resistance at the high temperature is formed into coating layers on the opposite surfaces in order to reduce the cost. It has heretofore been considered from common sense that when the opposite surfaces are hard, the wear is little. However, it is seen from this result that the wear is excessively large simply by the hardening, and the treatment cannot be applied.

Treatment number 4 shows one example of the present invention in which the Stellite-based material having a large content of Cr and having the lubricating property at the high temperature is applied to the opposite surfaces by the electro-discharge surface treatment in the same manner as in 3. In this case, it is seen that the wear amount largely drops and the treatment is applicable.

Furthermore, upper and lower test pieces are subjected to the electro-discharge surface treatment with the Stellite-based lubricating material in the porous manner (i.e., in a manner forming a porous structure), and the solid lubricating material which is chromium oxide (Cr₂O₃) or a mixture of chromium oxide (Cr₂O₃) and silicon is mixed with a soft rubber and filled in the pores by rubbing. Accordingly, the lubricating property can be enhanced even at 700° C. or less. Here, the porosity of the porous structure is preferably 10% or more.

As one example, when the pores are filled with solid lubricating material Cr₂O₃, there is the lubricating property, and the material is not worn regardless of the temperature. When the solid lubricating material of Cr₂O₃ mixed with Si is filled in the pores, the wear is remarkably little at 480° C. Si is oxidized to form SiO₂, and liquefied glass and solid lubricating material Cr₂O₃ produce the synergistic effect.

On the other hand, the porous structure has a small tensile strength and has a possibility of peel. To solve this, the structure is heated at the high temperature to promote the diffusion among the particles, and the coupling strength is increased.

As one example, when the layers of the electro-discharge surface treatment are heated/treated at 1000° C. or more for ten minutes or more, the diffusion coupling of the structure in the coating advances, and the densification and strength enhancement of the structure are realized.

Treatment number 5 shows another example of the present invention in which one surface is subjected to the electro-discharge surface treatment with very hard chromium carbide (Cr₃C₂), and on the other surface, the build-up material T having the lubricating material is build-up welded. As a result, it has been found that the wear amount is equal to or less than that of the treatment number 2, and the cost can be reduced by omitting the welding with respect to one surface.

Treatment number 6-1 shows another example of the present invention in which only the hard material of cBN is applied to the opposite surfaces in the electro-discharge surface treatment in the same manner as in 3. With cBN which does not have the lubricating property but which is excessively hard, it is observed that the coefficient of friction is large and the vibration at the time of sliding movement is large, but it is seen that the wear amount largely drops and the treatment is applicable.

Treatment number 6-2 shows another example of the present invention in which only the hard material of TiC is applied to the opposite surfaces in the electro-discharge surface treatment in the same manner as in 3. Although any lubricating material is not used, the coefficient of friction is small, and the hardness is remarkably large. Therefore, it is seen that the wear amount largely drops and the treatment is applicable. It has heretofore been considered that TiC is oxidized at 700° C. or more and is not usable at the high temperature. However, as a result of the high-temperature wear test, it has been seen that in the coating of the mother material of an Ni alloy with TiC, the wear is small and the high-temperature wear resistance is superior even at 1000° C. Additionally, the wear is slightly large at 480° C.

Table 7 shows the high-temperature wear amount of TiC.

TABLE 7 Wear test result Wear with respect to test temperature (surface pressure = 7 MPa, amplitude = ±0.5 mm, frequency = 40 Hz, cycle number = 10⁶) Base material: Re77 (Nickel alloy) unit: μm Build-up welding of EDC of TiC build-up material T (thickness of ° C. Re77 (thickness of 1 mm) 0.03 mm) 480 40 181  55*1 700 13 17 815 750  13 14 1000  Remarkably 3 25 large Coefficient of 07 04 0.1 to 0.2 friction *1The coating is worn and eliminated.

Treatment numbers 7, 8 show the other examples of the present invention in which a mixture of hard materials of hard Cr₃C₂ and cBN having good resistance to oxidation at the high temperature, and a Cr-containing material having the lubricating property at the high temperature is applied to the opposite surfaces in the electro-discharge surface treatment in the same manner as in 3. Although Cr₃C₂ of the treatment number 7 is hard and contains the high-temperature lubricating material, the material is worn. It has been seen that cBN of the treatment number 8 is remarkably hard and also contains the high-temperature lubricating material, therefore the wear amount largely drops, and the treatment is applicable.

In this manner, since the use of a usual lubricating material for use at room temperature is impossible under the high-temperature environment, a measure concerning the wear resistance is insufficient even with the electrode containing only the material having the strength. It has been seen that the effect against the wear can be produced when the material having the strength under the high-temperature environment in the components of the electrode for the electro-discharge surface treatment contains the material capable of fulfilling the lubricating property.

It is to be noted that in the above-described test examples, the examples of the electrode for the electro-discharge surface treatment blended with powders of Cr₃C₂+Cr+Co and cBN+Cr+Co have been described, but any material may also be used as long as the material having the high-temperature strength is combined with the material indicating the lubricating property at the high temperature.

Moreover, Cr is mixed in the electrode for the electro-discharge surface treatment as the material indicating the lubricating property at the high temperature in the above-described test examples, but, needless to say, the solid lubricating materials such as hBN may also be mixed instead. The mixture of hBN directly in the electrode is effective for lubrication in a range at a temperature at which a usual lubricant is usable or higher and a temperature at which Cr is oxidized at the high temperature or lower. When Cr and hBN are simultaneously mixed as the material of the electrode for the electro-discharge surface treatment, the coating layer having the lubricating property can be formed over a broad range from a comparatively low temperature to a high temperature exceeding 700° C.

Furthermore, some examples of the high-temperature hard material have been described. Above all, Cr₃C₂ has a function of remarkably improving roughness of the surface of the formed coating layer as compared with the use of the other materials, and Cr₃C₂ is preferably used as the high-temperature hard material. Since the coating layer formed by the electro-discharge surface treatment is formed by pulse electro-discharge in the dielectric liquid, there is a strong tendency to convex/concave surface, but it is possible to inhibit the surface roughness and to smoothen the surface by the use of Cr₃C₂.

As described above, in accordance with the method and high-temperature member of the present invention, when the sliding surface is subjected to the electro-discharge surface treatment with the high-temperature hard material (4), the hardness of the sliding surface can be enhanced. When the sliding surface is subjected to the electro-discharge surface treatment with the material (6) having the lubricating property at the high temperature, the lubricating property of the sliding surface at the high temperature can be enhanced. Furthermore, when both the high-temperature hard material (4) and the material (6) having the lubricating property at the high temperature are used in the electro-discharge surface treatment, the hardness and the high-temperature lubricating property can simultaneously be enhanced.

Moreover, in the electro-discharge surface treatment, the adhesion of the coating layer is equal to that in the welding, the dimensional precision and quality are stabilized and the workability is high as compared with the welding or the thermal spraying, and therefore the manufacturing cost can largely be reduced.

Furthermore, the treatment can easily be applied even to the narrow trench-shaped portions such as the fitting portion only by the change of the electrode shape in order to form the coating layer by the electro-discharge.

Additionally, when any or a mixture of cBN, TiC, TiN, TiAlN, TiB₂, WC, Cr₃C₂, SiC, ZrC, VC, B₄C, Si₃N₄, ZrO₂, and Al₂O₃ is used in the high-temperature hard material (4), the high-temperature hardness of the sliding surface of the high-temperature member can largely be enhanced.

Moreover, by the use of the material containing chromium and/or chromium oxide and/or hBN having the lubricating property at the high temperature, the lubricating property is fulfilled at the high temperature, and the resistance to wear can be enhanced to obtain the resistance which is equal to or more than that of the related art.

As described above, the coating method of the sliding surface of the high-temperature member and the high-temperature member of the present invention have superior effects that the wear is little even with the sliding movement under a high surface pressure at the high temperature, the adhesion, dimensional precision, workability of the coating layer are enhanced, the treatment by the electro-discharge is controlled by NC, and the automation is possible.

Next, preferable embodiments of the electrode for the electro-discharge surface treatment according to the present invention will be described in detail.

In the present invention, for the coating layer formed on the surface of the workpiece by the electro-discharge surface treatment method, the electrode material having the high strength and lubricating property even under the high-temperature environment is used.

Concretely, the high-temperature hard material having the high strength under the high-temperature environment, selected from cBN, TiC, TiN, TiAlN, TiB₂, WC, Cr₃C₂, silicon carbide (SiC), ZrC, VC, B₄C, Si₃N₄, zirconium oxide (ZrO₂), and aluminum oxide (Al₂O₃) is mixed with the high-temperature lubricating material which is oxidized under the high-temperature environment to obtain the lubricating property to form the electrode material.

As described above, especially Cr is preferably used as the material having the lubricating property under the high-temperature environment in the present invention. This is because Cr₂O₃ obtained by the oxidation of Cr has a property that the lubricating property is possessed under the high-temperature environment. Additionally, there are conditions for obtaining Cr₂O₃ having the lubricating property by the oxidation of Cr. In an environment where much Ni exists around Cr, Cr does not turn to Cr₂O₃, and therefore the lubricating property is not developed under the high-temperature environment. Moreover, when Cr is oxidized to form Cr₂O₃, a temperature is required at about 700° C. or more. Since Cr contained in the coating layer is not oxidized at a lower temperature, the effect indicating the lubricating property cannot be anticipated. Therefore, the lubricating property by the mixture of Cr into the electrode can be anticipated for the portion (case) for use at the high temperature. FIGS. 10, 11, 12, 13 show data as a proof for this fact. Conversely, a usual liquid lubricant evaporates in the high-temperature environment at 700° C. or more. The usual chemically synthesized lubricant is decomposed by heat, and is not usable, and therefore the lubricating effect by Cr₂O₃ has a significant meaning.

Next, for the electrode for the electro-discharge surface treatment of the present invention, the use of cBN which is the high-temperature hard material and Cr which is the high-temperature lubricating material will be described in accordance with an example.

FIG. 17 is a flowchart showing a preparation procedure of the electrode for the electro-discharge surface treatment according to the present invention. First, cBN and Stellite powder are weighed and mixed at a weight ratio of 1:4 (step S1), and charged in a mold (die) having a predetermined shape and subsequently compressed/molded to obtain a green compact (step S2). It is to be noted that Stellite is an alloy containing Co and Cr which are major components and also containing several types of other components. cBN having a particle diameter of 2 to 4 μm, and Stellite having a particle diameter of 1 to 3 μm were used.

FIG. 18 is a diagram showing a sectional shape of a molding unit for use in compression molding. A lower punch 104 is inserted via a lower part of a hole formed in a mold (die) 105, and a mixture of mixed cBN powder 101 and Stellite powder 102 is charged in a space formed by the lower punch 104 and the mold (die) 105. Thereafter, an upper punch 103 is inserted from an upper part of the hole formed in the mold (die) 105. In this manner, the material is compressed/molded with a pressurizing unit, and the like so that a pressure is applied on the opposite sides of the upper punch 103 and lower punch 104 of a molding unit charged with the mixture of the cBN and Stellite powders, and accordingly the electrode for the electro-discharge surface treatment is prepared. It is to be noted that, if necessary, it is possible to heat/treat the compressed/molded green compact (step S3). By the heating treatment, the strength of the compressed/molded electrode can be increased.

When the cBN powder is mixed with the Stellite powder in the above-described step S1, the wax may also be mixed. After mixing the wax into the powder in this manner, the material is compressed/molded, and the moldability of the green compact is then enhanced. Additionally, the wax is an insulating material. Therefore, when a large amount of wax is left in the electrode, an electric resistance of the electrode is increased, electro-discharge properties are degraded, and it is therefore necessary to remove the wax. As a method of removing the wax, there is a method in which a compressed/molded green compact electrode containing the wax is charged and heated in a vacuum furnace. Accordingly, the wax in the green compact is removed. It is to be noted that the method of the heating in the vacuum furnace corresponds to the process of the step S3. When the material is heated as described above, the strength of the electrode increases.

FIG. 19 is a block diagram showing an outline of an apparatus for performing the electro-discharge surface treatment method of the present invention. The electro-discharge surface treatment apparatus includes an electrode for the electro-discharge surface treatment 21, processing tank 24, dielectric liquid 25, resistance 26, power supply 27, switching device 28, and control circuit 29. The dielectric liquid 25 mainly containing an insulating oil or water is charged in the processing tank 24, and a workpiece 22 on which the coating layer is to be formed is disposed. The electrode for the electro-discharge surface treatment 21 is the electrode prepared in the above-described steps of FIG. 17, and is disposed opposite to the upper part of the workpiece 22. The electrode for the electro-discharge surface treatment 21 is connected to the workpiece 22 via the resistance 26, power supply 27, and switching device 28. The switching device 28 switches the voltage and current applied to the electrode for the electro-discharge surface treatment 21 and workpiece 22. The switching device 28 is connected to the control circuit 29 which controls the switching device 28 to be on/off. It is to be noted that although not shown, a driving mechanism is disposed at the electrode for the electro-discharge surface treatment 21 and processing tank 24 in order to control the position of the coating layer formed on the surface of the workpiece 22.

When the pulsed electro-discharge is generated between the electrode for the electro-discharge surface treatment 21 of the electro-discharge surface treatment apparatus and the workpiece 22, the material constituting the electrode for the electro-discharge surface treatment 21 floats in the dielectric liquid of the processing portion by heat energy. The floated electrode constituting material or the material obtained by the reaction of the electrode constituting material by the heat energy adheres to the surface of the workpiece 22 to form a coating layer 23. The coating layer 23 is formed on the surface of the workpiece 22 in this manner.

It is to be noted that the present invention has been described in accordance with several preferable embodiments, but it would be understood that the scope included in the present invention is not limited to these embodiments. On the contrary, the scope of the present invention includes all improvements, modifications, and equivalents included in the appended claims.

The method in which the electrode is formed by compression molding of powder using a press is described in the above. However, it is apparent that the method for forming the electrode is not limited to the compression molding as long as the electrode is formed by using powder. As a method for forming the electrode, there are a method that uses slip, a method that uses an MIM (Metal Injection Molding), a method that uses thermal spraying, a method that uses nano powder accompanying a jet stream, and the like. In the method that uses slip, powder is dispersed in a solvent, and the solution is put in a porous mold such as gypsum to remove the solvent so that the electrode can be molded. In the method that uses the MIM, a mixture of powder and binder is kneaded, and then injected into a heated mold. In the method that uses thermal spraying, heated powder is sprayed to combine a part of the sprayed powder, forming the electrode. These methods are different in a manner of forming the electrode, but a purpose of these methods is the same in terms of forming the electrode by using powder. In other words, when the combination of powder reaches a desired state, the combined powder can be then used as the electrode. 

The invention claimed is:
 1. A method for coating a sliding surface of a first member so that the first member is adapted to contact a second member under a high temperature environment of 700° C. or more, the method comprising the steps of: (a) providing an electrode for use in an electro-discharge surface treatment, wherein the electrode is a green compact that includes a constituting material that includes a first coating material that contains 10% or more of chromium, by weight, and does not contain 20% or more of nickel, by weight, or a mixture of the first coating material and a second coating material that is different from the first coating material, wherein the first coating material comprises the chromium that oxidizes at a high temperature of 700° C. or more at an oxidation amount sufficient to form Cr₂O₃ as a first lubricating material, and wherein the first coating material is a Co-based alloy; (b) performing the electro-discharge surface treatment including causing a pulsed discharge between the electrode and the sliding surface of the first member so that electro-discharge energy melts a portion of the constituting material of the electrode during electro-discharge surface treatment; (c) forming a porous coating layer made of the constituting material of the electrode during the electro-discharge surface treatment, or of a reactant of the constituting material, on the sliding surface of the first member using the electro-discharge energy, wherein the second coating material is any one or a mixture of cBN, TiC, TiN, TiAlN, TiB₂, WC, Cr₃C₂, SiC, ZrC, VC, B₄C, Si₃N₄, ZrO₂, and Al₂O₃; and (d) keeping the porous coating layer porous at least until the first member is disposed such that the sliding surface of the first member contacts the second member, wherein after the porous coating layer is formed on the sliding surface of the first member, the first member is disposed so that the porous coating layer of the sliding surface of the first member contacts the second member, and wherein the porous coating layer of the sliding surface contacts the second member under the high temperature environment of 700° C. or more and the chromium of the porous coating layer oxidizes at 700° C. or more of the high temperature environment to increase formation of the first lubricating material.
 2. The coating method according to claim 1, wherein the second coating material is a hard material having a Vickers hardness of between 1300 and
 4500. 3. A method for coating a sliding portion, the method comprising the steps of: (a) subjecting a sliding surface A of a first member to an electro-discharge surface treatment using an electrode, wherein the electrode includes a constituting material that includes a first coating material that contains 10% or more of chromium, by weight, and does not contain 20% or more of nickel, by weight, or a mixture of the first coating material and a second coating material that is different from the first coating material, wherein the first coating material comprises the chromium that oxidizes at a high temperature of 700° C. or more at an oxidation amount sufficient to form Cr₂O₃ as a first lubricating material, and wherein the electro-discharge surface treatment is a treatment comprising the step of generating a pulsed electro-discharge between the electrode used for electro-discharge surface treatment and the sliding surface A of the first member in a dielectric liquid or gas so that the constituting material of the electrode, or a reactant of the constituting material, forms into a porous coating layer on the sliding surface A of the first member due to electro-discharge energy, and wherein the first coating material is a Co-based alloy; (b) subjecting a sliding surface B adapted to abut on the sliding surface A to electro-discharge surface treatment using the first coating material, or to a welding treatment, or to a thermal spraying treatment, wherein the second coating material is any one or a mixture of cBN, TiC, TiN, TiAlN, TiB₂, WC, Cr₃C₂, SiC, ZrC, VC, B₄C, Si₃N₄, ZrO₂, and Al₂O₃; and (c) keeping the porous coating layer porous at least until the first member is disposed such that the sliding surface A of the first member contacts the sliding surface B, wherein after the porous coating layer is formed on the sliding surface A of the first member, the first member is disposed so that the porous coating layer of the sliding surface A of the first member and the sliding surface B abut, wherein the sliding surface B abuts the sliding surface A of the first member under the high temperature environment of 700° C. or more and the chromium of the porous coating layer oxidizes at 700° C. or more of the high temperature environment to increase formation of the first lubricating material.
 4. The coating method according to claim 3, wherein the second coating material is a hard material having a Vickers hardness of between 1300 and
 4500. 5. A method for coating a sliding portion, the method comprising the steps of: (a) subjecting a sliding surface A of a first member to electro-discharge surface treatment using a first coating material that contains 10% or more of chromium, by weight, and does not contain 20% or more of nickel, by weight, or a mixture of the first coating material and a second coating material that is different from the first coating material, wherein the first coating material comprises the chromium that oxidizes at a high temperature of 700° C. or more at an oxidation amount sufficient to form Cr₂O₃ as a first lubricating material, and wherein the first coating material is a Co-based alloy; (b) forming a porous coating layer made of constituting material of an electrode used during the electro-discharge surface treatment or of a reactant of the constituent material on the sliding surface of the first member using the electro-discharge energy, wherein the constituting material of the electrode comprises the first coating material or the mixture of the first coating material and the second coating material; (c) subjecting a sliding surface B of a second member to electro-discharge surface treatment using the first coating material, the second coating material, or the mixture of the first coating material and the second coating material, wherein the sliding surface B of the second member is adapted to abut the sliding surface A of the first member; and (d) keeping the porous coating layer porous at least until the first member is disposed such that the sliding surface A of the first member contacts the sliding surface B of the second member, wherein after the sliding surface A of the first member and the sliding surface B of the second member are respectively subjected to the electro-discharge surface treatment, the sliding surface B of the second member is disposed to abut the porous coating layer of the sliding surface A of the first member under the high temperature environment of 700° C. or more and the chromium of the porous coating layer oxidizes at 700° C. or more of the high temperature environment to increase formation of the first lubricating material.
 6. The coating method according to claim 5, wherein the second coating material is a hard material having a Vickers hardness of between 1300 and
 4500. 